Indian Journalof Chemistry Vol.39A, July 2000, pp.703-708

A study on equilibrium and kinetics of carbocation-to-carbinol conversion for di- and tri- arylmethane cations in aqueous solutions: Relative stabilities of dye carbocations and mechanism of dye carbinol formation

Susanta K Sen Gupta', Sangeeta Mishra & V Radha Rani Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India

Received 17 August 1999; revised 7 April 2000

Arylmethane dye cations form a structurally interesting set of stable carbocations. A detailed study on rate-equilib­ ria of carbinol formation from two diarylmethane and nine triarylmethane dye carbocations in aqueous solutions has been carried out using spectrophotometric measurements. The conclusions reached are : (i) The stability order found (auramine O>crystal

violet = > victoria blue R > victoria pure blue BO = ethyl violet > > brilliant green > > carbocation form of Michler's hydrol > methyl green), seems to be determined by an interplay of dye carbocation / carbinol conformation and stereoelectronic effects of substituents; and (ii) carbinol formation is general base catalysed and occurs by the rate determining attack of a HzD molecule on the dye carbocation centre via two kinetic pathways one mediated by another H 0 molecule and the other by a OR ion. 2

Rate-equilibrium parameters and structure-reactivity libriJ.lmof arylmethane dye carbocation-to-carbinol con­ correlations for carbocation-to-carbinol conversion pro­ version in aqueous media was reported excepting the vide a valuable basis for the analysis of mechanisms of instances of and malachite greenH•9 and of reactions involving carbocation intermediates. One some preliminary studies on carbinol formation from structurally interesting class of carbocations is provided several such dyes10.13; whereas carbinol formation from by 4-amino or alkylamino arylmethane dye cations. various other carbocations has been the subject of many 2 Highly electron donating 4-aminoaryl groups endow a systematic investigations�·9.11.1 .19. To wards bridging this methyl cation remarkable stability in aqueous solutions. gap a detailed investigation on rate-equilibrium of dye Further, steric crowding by aryl groups around the cen­ carbocation-to-carbinol conversion in aqueous solutions tral carbon makes reactions of these methyl cations with was undertaken with the objectives of determining sta­ a nucleophile considerably slower than those of most of bility scales, structure-reactivity relationships and other cations of similar stability. In fact, the rate of con­ carbinol-formation mechanisms for a set of eleven di-/ version of 4-amino or alkylamino di- or tri-arylmethane tri- arylmethane dye·cations. dye carbocation to the carbinol is sufficiently slow for monitoring the kinetics by conventional spectrophoto­ , The selected dye cations are : Carbocation form + metric techniques. Moreover, a significant number of of Michler's hydrol (MH; 4,4'-bis NMe2 Ph2C H), + structural effects due to carbocation structure, auramine 0 (AR; 4,4'-bis NMe2 Ph2C NH2), pararosa- " + stereoelectronic influences of aryl ring substituents, hy­ . nHine (PR; 4,4',4 -tris NH2 Ph]C ), malachite green + ' dration of the substituent and the carbocation centre etc. (MG; 4,4'-bis NMe2 Ph3C ), brilliant green (BG; 4,4'­ + are encountered in the reaction. Arylmethane dy e cat­ bis NEt2 Ph3C ), methyl violet (MV; 4,4'-bis NMe2 - " + " ions are well-known reagents for solvent extracti<:>n-spec­ 4 -NHMe Ph3C ), crystal violet (CV; 4,4',4 -tris NMe2 + " + trophotometric determination of a number of metals and Ph,C" ), methyl green (MeG; 4,4'-bis NMe2 -4 -N Me2 Et 2 Ph C+), " + non-metals 1. . Their carbinol forms are sufficiently ba­ 3 ethyl violet (EV; 4,4',4 -tris NEt2 Ph C ), " 3 sic and potential reference bases for studying proton victoria blue R (VBR; 4,4'-bis NMe2 Ph2-4 -NHEt 1- + transfer from various acids in apolar aprotic solvents3.? NaphC ), and victoria pure blue BO (VBP; 4,4'-bis NEt2 " + Despite such importance no detailed study on rate-equi- Ph2 -4 -NHEt I-NaphC ). 704 INDIAN J CHEM. SEC. A, JULY 2000

Materials and Methods BG, 5.46 fo r MV, 5.19 for CV, 6.13 for MeG, 6.03 for The di- and tri- arylmethane dye samples employed EV, 5.27 for VBR and 2.70 for VBP at their respective were from Aldrich, BDH, E. Merck or Sigma. Ten of A the eleven samples were obtained in their cationic (R+) max. form viz. auramine 0, pararosaniline, malachite green, Results and Discussion brilliant green, methyl violet, crystal violet, methyl green, Dye carbocation - carbinol equilibration ethyl violet, victoria blue R and victoria pure blue BO. However, the remaining sample was obtained in its The equilibrium constant (KR +) for dye . + . colourless carbinol (ROH) from Michler's hydro 1. It was carb ocatlOn (R )-to-carbmol (ROH) conversion in aque- completely converted to its coloured cationic from (MH) ous solutions: by dissolving in a HCI solution of pH -5 and adjusting ROH + H+ the pH for the maximum absorbance of the cationic form . ..(1) of Michler's hydrol (MH) at its , 604 nm. Am ax . ..(2) The buffers used for carbocation-carbinol conversion (the activity of water being defined as unity in dilute for the above were phosphate (KHl04-NaOH) aqueous solutions) was determined as : buffers for pH, 5.50-9.00 and borate (Na B 4 0 -NaOH) buffers for pH, 9.20-11.60. pH was measured2 7 in a 335 ... (3) Systronics digital pH-meter. + where All and Ac are absorbances of the dye cation (R ) initially and at equilibrium at its The Debye-Hiickel Equilibrium and kinetics of carbocation-to-carbinol A.nax conversion for a dye cation (R+ ) were studied by mea­ correction for activity coefficients, -Alz+llzt'-'flat the ionic suring absorbances of the dye cation in buffer solutions strength (fl) of 0.0 1 M for the buffer media used was not of appropriate pH range each maintained at the ionic significant enough to be incorporated in Eq.3. Values of strength of O.OIM, at 35.0 ± O. loC for an initial dye cat­ pKR+ for the select dye carbocations are reported in ion (R+) concentration (-4 x 1O.5M) corresponding to Table I. the absorbance of Abs. at its measuring wave length As seen from the data in Table I, stability of the dyes I measured by their pKR varies considerably spanning a (A ) in a 160 A Shimadzu UV-vis recording spectro­ + photometerm with cuvet thermostatting facility. The ab­ range of 4.4 pKR+ units and depends significantly on sorbance of a dye cation (R+) falls with increase in pH the type and number of aryl rings contiguous to the due to increase in conversion to the colourless carbinol carbocation centre as well as on the nature and number of -donor substituents at the 4-position of the aryl rings. (ROH). The initial absorbance (Ao) for the calculation 1t of equilibrium and rate constants (Eqs 3 and 6) was then The order of stability among the dye carbocations in aqueous solutions is thus : the limiting maximum absorbances at its A on lower- ing the pH of the medium. The pHs chosen maforx the study + AR > CY MY > YBR > YBP EY > PR > BG> MG > MH > were not on the strongly acidic side so that R -ROH = = MeG ...(4) equilibration was freed from participation of species like RH2+, RHOH+. Absorbances were measured at 604 nm The effects of structure on dye carbocation stability for MH, 540 nm for PR, 615 nm for MG, 630 nm for Stability of a dye cation towards a nucleophilic at­ BG, 580 nm for MV, 590 nm for CV, 632 nm for MeG, tack should depend primarily upon the degree of direct 595 nm for EV, 610 nm for VBR and 615 nm for VBP. resonance interaction between its cabocation centre and However, for AR which undergoes conversion to both the lone pair of electrons on the nitrogen atom of 4-aminol carbinol and imine forms, measurements of absorbance alkylamino substituents on the aryl rings attached to it. were made simultaneously at three wavelengths : 315 A perusal of the dataonpK + and (sum of the Brown nm, 370 nm and 430 nm corresponding to imine form, R I,cr+ substituent constants 14.15 expressing exalted resonance carbinol form and cationic form of the dye AR respec­ properties for the 4-substituents over all the aryl rings) tively (discussed in Appendix).The molar absorption in Table 1 shows clearly that there is no overall general coefficients (104 dm3 mol·1 cm·l) for the dyes were : 9.53 correlation between pK + and e.g. although the set for MH, 2.86 for AR, 6.46 for PR, 6.24 for MG, 5.07 for R I,cr+ Table 1- Equilibrium and kinetic parameters for dye carbocation-to-carbinol conversion at 35�. 1 °C

Dye carbocations + Log k Log k Chemical name 4-aryl ring aLO pH range pKR + OH H2 substituents b(±O.03) (M·lmin·l) (min·I) 0 (±O. I) (±O. I) b O b O Cation form Michler's hydrol (MH) 4,4'-bis dimethylamino diphenyl carbocation Two-NMe2 -3.40 5.72-7.20 5.92 7.34 -0.76

en Auramine 0 (AR) 4,4'-bis dimethylamino diphenylamino carbocation Two-NMe2 -3.40 9.80-11.00 9.88 0.13 -2. 10 tTl Z Pararosaniline (PR) 4,4',4"-tris amino triphenyl carbocation Three-NH2 -3.90 7.00-9.06 7.27 2.64 -1.70 Q C Malachite green (MG) 4,4'-bis dimethylamino triphenyl carbocation Two-NMe2 -3.40 6.50-8.02 6.55 2.66 -1.69 � Brilliant green (BG) 4,4'-bis diethylamino triphenyl carbocation Two-NEt2 -4. 14 6.32-8.34 6.87 2.50 -1.87 � f.?- Methyl violet (MV) 4,4'-bi s dimethylamino 4" -mono methyl amino Two-NMe2 and -5.21 8.70-10.20 9.02 1.42 -3.02 tTls::: n triphenyl carbocation One-NHMe :r: ;I> Crystal violet (CV) 4,4',4"-tris dimethylamino triphenyl carbocation Three-NMe2 -5.10 8.70-10.10 9.06 1.40 -3.04 Z v:; Methyl green (MeG) 4,4'-bis dimethylamino s::: Two-NMe2and -2.99 5.50-7.50 5.48 3.31 -1.51 0 4"-dim ethyl ethyl ammonium triphenyl carbocation 'Tl -<0 One-N+Me2Et tTl n Ethyl violet (EV) 4,4',4"-tris diethylamino triphenyl carbocation Three-NEt2 -6.21 8.20-10.00 8.56 1.68 -2.80 ;I> :;0 t:J:! Victoria Blue R (VBR) 4,4'-bis dimethylamino diphenyl, Two-NMe2 and Z 4" -monoethylamino I-napthyl carbocation One-NHEt -5.21 9.30-10.60 8.67 1.18 -2.86 0 r Victoria Pure Blue BO (VBP) 9.60-11.00 8.57 1.73 -2.76 'Tl 4,4'-bis diethylamino diphenyl, 4"-monoethylamino Two-NEt2 and -5.95 0 I-naphthyl carbocation One-NHEt :;0 s::: � Z0 a: cr+ for -NH2 -1.30, -NHMe -1.81, -NMe2 -1.70, -NEt2 = -2.07 (vide Ref. 15.). = = = cr+for -N+Me2Et being not available taken equal to o- for -N+Me3 = +0.41 (vide Ref. 15.). cr+for -NHEt being not available taken equal to cr+for -NHMe = -1.80 on the basis of nearly equal dipole moments (-I.64D)of N-methyl and N-ethylanilines (vide Ref. 16). The substituent constant would be more negative for a I-naphthyl ring, its conjugating power being 1.6 times that of a phenyl ring (vide Dewar, M J S, The molecular orbital theoryof organic chemistry,McGraw Hill, New York, 1969). b : Maximum variation

-.J o \Jt 706 INDIAN J CHEM, SEC. A, JULY 2000

12.00 r------...... , thus be attributed to lack of coplanarity of their 4- 11.50 11.00 ethylamino substituents with the respective aryl rings. 10.50 The markedly lower of the diphenylmethyl cat­ 10.00 pKR+ 9.50 ion MH as compared to its norm value which is coinci­ PK +9.00 8.50 R dent with pK + of its phenyl derivative MG (Fig. 1) is, 8.00 R 7.50 however, difficult to rationalize. On going from MH to 7.00 6.50 MG, distortion of a planar structure to a propeller-type 6.00 enforced by severe steric interactions between ortho pro­ 5.50 2 5.00 L-....______tonsl would give rise to significant differences in elec­ :::::...... J 2 � � � � � - � � � � tronicI .IK.19, stericll, hydrationll etc. effects between these two carbocations and their carbinols. The net result can­ not be predicted without detailed knowledge of Fig.l- The norm pK versus La+ relationship (Eq.5) for dye a priori R+ conformations of the carbocations and the carbinols. carbocation-to-carbinol equilibration; and the points in pKR + versus La+ plot for eleven di-/tri-arylmethane dye The other diphenylmethane dye cation AR unlike the cations(1:MH, 2: AR, 3:PR, 4:MG, 5:BG, 6:MY,7:CY, above six dyes shows a positive deviation from the norm 8:MeG, 9:EY, IO:YBR, II:YBP) (Fig. I). AR has the peculiarity of having a -NH2 group of dye cations (MH, AR and MG) or (MV and VBR) bonded to its central carbon. Its too high pKR+ can thus have same their values differ considerably be attributed to delocalization of the carbocationic charge Icr+, pKR+ pointing to significant effect of dye carbocation struc­ by strong resonance effect of the -NH2 group. + + + ture(-Ph 2 C H,-Ph 2 (NH2 )C ,-Ph C or + 3 Kinetics and mechanism of dye carbocation-carbinol -Php-Naph)C ) on pKR+. However, within a set of four triphenylmethane dye cations PR, MG, MV and CV equilibration + (i.e. those having -H, -NH2, -NHCH or -N(CH )2 group The dye carbocation (R ) -carbinol (ROH) equili­ • 3 3 ' only as 4-substituents ) there exists a linear pKR+ versus bration in a buffer of given pH for each of the eleven correlation ( r 0.996) : select dyes was found to follow first order kinetics. The Icr+ = overall pseudo-first order rate constant determined (k), ...(5) using Eq. (6)

Since of all 4-substitutents in triarylmethane dye cat­ k (2.303/t) log (A/A) ... (6) = ions, only -NH2 -NHCH and -N (CH)2 do not interact ' 3 where and are absorbances of the dye carbocation sterically with their ortho- or peri- hydrogen and would Au At exert their characteristic direct resonance effect for a at its A x initially and at time t, was found to increase linearlyma with [OH-]. The overall rate constant for triarylcarbocation centre, the correlation line (Eq. 5) for (k) the dyes PR, MG, MV and CV can justifiably be re­ the equilibration is garded as the norm for dye carbocation-to-carbinol equili­ (7) bration. Figure I displays this norm and shows fu rther ... that the remaining five triarylmethane dyes (i.e. BG, EV, where and are the individual pseudo-first order rate MeG, VBR and VBP) and one diarylmethane dye (MH) kf k constants for hthe forward step, the formation of ROH have too low pK + whereas the other diarylmethane dye R from R+ and the reverse step, the decomposition of ROH (AR) has too high pK + than what be predicted from the R back to R+, respectively of the R+-ROH equilibration. norm. These five triarylcarbocations have one or more 4-NHC H 4-N(C H ) 0r 4-N+(CH ) C H substituents 2 s' 2 s 2 3 2 2 S At equilibrium, each in their phenyl or I-naphthyl rings. However, these 4-substituents are not coplanar with their aryl rings ow­ ... (8) ing to steric interactions with the ortho- or peri- hydro­ gen 17 and thus would exert their resonance effect on the and applying Eq. (2), carbocationic site at a considerably reduced level as com­ ... (9) pared to a 4-NH2 4-NHCH or 4-N(CH )2 substituent. + ' 3 3 Too low pKR for BG, EV, MeG, VBR and VBP can It follows,

.. SEN GUPTA et at.: MECHANISM OF DYE CARBINOL FORMATION 707

... (10) nucleophile, on one hand water in the cybotactic region around the dye carcbocation gets more electrostricted as The results on pH-dependence of pseudo-first order the positive charge on its central carbon increases due to rate constant (kf) for carbinol formation from each of the accompanying attenuation of direct conjugation in the dye carbocations were found to fit the expression : carbocation, and on the other hand 'highly disordered' 2 ...(11) water 0 in the shell exterior to the cybotactic region is released to 'less disordered' bulk water both contribut­ Thus, dye carbinol formation isgeneral base catalysed ing to the negative entropy of activation. The body of and appears to occur by two parallel kinetic routes. The evidence so obtained suggests interpretation of kOH in - first one distinguished by the pseudo-first order rate con­ terms of a transition state in which OH instead of at- stant kH 0 is interpretable as the rate-determining attack tacking directly the dye cation, acts as a general base of a H > molecule on the dye carbocation mediated by catalyst for the attack of a H20 molecule on the l carbocation, essentially a solvent-separated ion-pair, the another H20 molecule as the general base catalyst. The other one distinguished by the second order rate con­ transition state postulated by Ritchie9 for cation-anion stant kOH ' the predominant route in basic solution, is combination reactions ; and of kH 0 in terms of a transi- . • 2 interpretable as the OH- (general base catalyst)-medi­ tlon state where the nucleophilic attack of one H 20 mol- ated attack of a Hp molecule on the carbocation. The ecule instead of occurring simply, is assisted by general kinetic parameters kH 0 and kOH can reasonably be em­ base catalysis from a second Hp molecule. Very simi­ ployed as measures of relative electrophilicities of dye lar conclusions about the transition states of the reac­ carbocations. The values obtained for log kH 0 and log tions of Hp and OH- with quaternaryheterocyclic cat­ 2 kOH of the dye cations are compiled in Table along with ions were reached by Bunting and his coworkers 1 . I pK + and R Lif. Acknowledgement A perusal of the data in Table 1 shows that there is no overall general correlation between stabilities (pKR+ ) The financial support extended by the UGC, New and electrophilicities (log kH 0 or log kOH) of the dye cat­ Delhi is thankfully acknowledged. ions : e.g., although MH and MeG differ in stability only by 0.4 pKR+ unit, the difference in their electrophilici­ Appendix ties is as large as 3.9 log kOH units. However, among the The case of auramine 0 - Auramine 0 (AR), 4,4'-bis nine triarylmethane dye cations, there do exist fairly lin­ + NMe2 Ph2C NH2 (A.max = 430 nm) owing to its -NH2 ear correlations between log kH 0 or log kOH and 2 pKR+ : group bonded to the central carbon undergoes simulta­ ' neous conversion to its imine form (ARI mine. ) ' 44, -bis log kH = -0.52 pKR+ + 1.68 (11 = 9, r = 0.882) ...(12 ) NMe Ph C=NH 315 nm) called Homolka basel{) 0 2 2 (A.max = 2 ' and its carbinol form (ARcarhlnol . ) ' 4 , 4 -bis NMe2 log kOH = -0.51 pKR+ + 6.18 (11 = 9, r = 0.847) ...(13 ) PhF(NH 2)OH (A.max = 370 nm) in basic buffers, the first order plot for decolorisation of AR monitored at 430 nm The equality in the regression coefficients of the Eqs was bilinear, the initial one of higher slope correspond­ (12) and (13) and the near unity regression coefficient ing to ARm formation supported by simultaneous moni­ of log kOH versus log kH 0 correlation : i inc 2 toring at 315nm, reaching equilibration within 80- 100 s, depending on pH, and the second slower one to AR . log k = 0.90 log + 4.22 (11 = 9, r = 0.853) ...(14 ) carhlnol OH • kH 0 2 formatIOn supported by monitoring at 370 nm. Thus if lend strong support to the inference drawn on the close and refer to absorbances of AR at 430 nm for a A", AT Ac similarity between the two carbinol formation mecha­ medium of given pH, corresponding to commencement nisms characterised by log kH 0 and log OH This is fur- of decolorisation of AR, completion of imine equilibra­ 2 k " ther supported by Ritchie et at. 's finding that entropies tion - the intersection point of the above bilinear plot of activation for carbinol formation from two and final equilibration for both ARImine. and ARcarhmol . for- •• triarylmethane dyes MG and CV are highly negative by mat !On respectIvely. both the pathways (log kH 0 and log kOHl. The solvent 2 pH + log A (A -A .>would measure pK with K defined cI o c water thus must be more ordered at the transition state . . + eqll eqll than at the reactant. It appears that on the approach of a as { [H ]([(AR"ni.,J+[AR""h'OM" j)/[AR] I 708 INDIAN J CHEM, SEC. A, JULY 2000

6. Sen Gupta S K & Arvind U, J phys org Chern, 6 (1993) 145. and pH log ATIt'Ao-AT) would measure pK;. ....with K;mino:defined + 7. Sen Gupta S K & Arvind U, J phys orgChern, 10 (1997) 466. as ([WHAR .....J;1AR] ). ; 8. Ritchie C D, Wright D J, Huang D S & Kamego A A, JAm chernSoc, 97 (1975) 1l63. KR+ as defined by Eq. 2 was then (K ) and 'I - Imine' eql K 9. Ritchie C D, Can J Chern, 64 (1986) 2239. reported as pK + in Table 1. Further, the value of pKi i R m nc 10. Goldacre R J & Phillips J N, J chern Soc, (1949) 1724. - pKR+ which equals log [ARcarbinnl] [ARi inJ showed / m Deno N C, Jaruzelsky J ] & Schriesheim A, J org Chern, 19 that the total colourless base of auramine 0 in equilib­ II. (1954) 155. rium with the dye cationis 89% in the form of the carbinol 12. Freedman H H, in Carbonium ions, Vol. 4, edited by G A OIah base, ARcarbinnl and 11% in the form of the imine base & Pv R Schleyer (Wiley-Interscience, New York), 1973, 1501. AR (values of pK R+ 13. Sen Gupta S K & Arvind U, lndian J Chern, 33B (1994) 998. imine imine, pK and pKeqil being 10.81, 9.88 and 9.85 respectively). 14. Brown H C & Okamoto Y, JAm chernSoc, 80 (1958) 4979. Hansch C, Leo A & Taft W, ChernRev, 91 (1991) 165. IS16. Chuchani G, in The chemistry of the amino group, edited by S References Patai (Interscience, London), 1968, 213. I. Marczenco A, MicrochirnAc ta II, (1977) 651. 2. Sato S, Anal chirnActa, 151 (1983) 465. 17. Smith ] W, in The chemistryof the amino group, edited by S 3. Bhowmik S R & Palit S R, J Indian chernSoc, 44 (1967) 1049. Patai (lnterscience, London), 1968, 187. 4. Sen Gupta S K & Balasubramanian 0, Bull chernSoc Japan, 64 18. Tsuno Y & Fujio M, Chern Soc Revs, 25 (1996) 129. (1991) 1437. 19. Olah G A & White A M, JAm chernSoc, 91 (1969) 5801. 5. Sen Gupta S K, Balasubramanian 0 & Arvind U, Indian J 20. Frank H S & Wen W Y, Discuss Faraday Soc, 24 (1957) 133. Chern,32A (1993) 810. 21. Bunting ] W, Adv heterocycl Chern, 25 (1979) I.

•